Modem electronic systems typically include a data storage device such as a dynamic random access memory (DRAM), static random access memory (SRAM) or other conventional memory device. The memory device stores data in vast arrays of memory cells. Each cell conventionally stores a single bit of data (a logical "1" or a logical "0") and can be individually accessed or addressed.
Electronic systems, e.g., computers, conventionally store data during operation in the memory device. As these systems become more sophisticated, they require more and more memory in order to keep pace with the increasing complexity of software based applications that run on the systems. Thus, as the technology relating to memory devices has evolved, designers have tried to increase the density of memory cells in the memory device. The electronics industry strives to decrease the size of the memory cells. This allows a larger number of memory cells to be fabricated without substantially increasing the size of the semiconductor wafer.
Static random access memory or "SRAM" is one type of memory device that is used with computers. Conventionally, an SRAM device includes an array of addressable memory cells. Each cell includes a four transistor flip-flop and access transistors that are coupled to input/output nodes of the flip-flop. Data is written to the memory cell by applying a high or low logic level to one of the input/output nodes of the flip-flop through one of the access transistors. When the logic level is removed from the access transistor, the flip-flop retains this logic level at the input/output node. Data is read out from the flip-flop by turning on the access transistor.
Memory devices are fabricated using photolithographic techniques that allow semiconductor and other materials to be manipulated to form integrated circuits as is known in the art. These photolithographic techniques essentially use light that is focussed through lenses to define patterns in the materials with microscopic dimensions. The equipment and techniques that are used to implement this photolithography provide a limit for the size of the circuits that can be formed with the materials. Essentially, at some point, the lithography cannot create a fine enough image with sufficient clarity to decrease the size of the elements of circuit. In other words, there is a minimum dimension that can be achieved through conventional photolithography. This minimum dimension is referred to as the "critical dimension" (CD) or minimum feature size (F) of the photolithographic process.
The minimum feature size imposes one constraint on the size of conventional SRAM memory cells. Conventionally, SRAM cells have used a surface area on a substrate that is approximately equal to 120 feature squares. Recently, researchers have designed an SRAM cell in an area of approximately 100 feature squares. In order to keep up with the demands for higher capacity memory devices, designers need to further reduce the size of the memory cells.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an SRAM cell which uses less surface area than conventional SRAM cells.